1. Field of the Invention
This invention relates to a semiconductor memory device and a memory card including the semiconductor memory device. More particularly, this invention relates to a nonvolatile semiconductor memory device including MOS transistors, each of which has a floating gate and a control gate.
2. Description of the Related Art
Various types of flash memories have been proposed for a variety of uses.
For instance, a NAND flash memory is known as a data storage memory for use in a digital camera or the like. A NAND flash memory is described in, for example, Imamiya K., et al., “A 125-mm/sup 2/1-Gb NAND Flash Memory With 10-Mbytes/s Program Speed,” IEEE Journal of Solid-State Circuits, Vol. 37, No. 11, November, 2002, pp. 1493–1501.
Another known flash memory is a NOR flash memory which writes and erases data by using FN tunneling. A NOR flash memory is described in, for example, Ditewig T., et al., “An Embedded 1.2-V Read Flash Memory Module in a 0.18-μm Logic Process,” Solid-State Circuits Conference, 2001 Digest of Technical Papers ISSCC. 2001 IEEE International 5–7, Feb. 2001, pp. 34–35, 425. The NOR flash memory is such that a memory cell has one memory cell transistor and two select transistors (hereinafter, referred to as a 3Tr-NAND flash memory).
Furthermore, in recent years, a flash memory combining the features of the NOR flash memory and the NAND flash memory has been proposed in, for example, Wei-Hua Liu, “A 2-Transistor Source-select (2TS) Flash EEPROM for 1.8-V-Only Application,” Non-Volatile Semiconductor Memory Workshop 4.1, 1997. A flash memory of this type has memory cells, each including one memory cell transistor and one select transistor (hereinafter, referred to as a 2Tr flash memory).
In the flash memory, when the data is written, a write inhibit voltage is applied to the unselected bit lines, thereby preventing electrons from being injected into the floating gates (or preventing erroneous writing).